The PCM (pulse-code-modulation) telecommunication system described in our copending application and patent comprises a modulation channel for converting low-frequency signals originating at a subscriber line into outgoing code pulses, a demodulation channel for converting incoming code pulses into low-frequency signals to be transmitted to that subscriber line, and duplexing means for coupling the two channels to the subscriber line while decoupling them from each other. An active filter inserted in at least one of the channels also functions as an amplifier either for the outgoing or for the incoming signals.